Electrical heater

ABSTRACT

An improved electrical heating unit useful as a surface cooking element and in a wide variety of other applications comprises a unitary fused silica body having a thermally conductive, normally upper layer consisting essentially of dense fused silica and, fused thereto, a thermally insulating, normally lower layer consisting essentially of closed cell cellular fused silica, one or more channels at the interface of the dense silica and cellular silica layers, and one or more electrical resistance heating elements disposed within said channels and preferably in contact with the dense silica layer.

United States Patent [191 Tymzcak [5 ELECTRICAL HEATER [75] Inventor: Romuald W. Tymzcak, Lewiston,

[73] Assignee: The Carborundum Company,

Niagara Falls, N.Y.

22 Filed: Dec. 16,1971

21 Appl. No.: 208,618

[52] US. Cl 219/540, 106/40 R, 219/464,

219/530, 219/552 [51] Int. Cl. H051) 3/06 [58] Field of Search 219/213, 345, 464,

[56] References Cited UNITED STATES PATENTS 2,007,111 7/1935 Morgan 219/540 X 2,010,768 8/1935 Morgan 219/540 3,396,043 8/1968 Winterburn.... 106/40 R 3,012,374 12/1961 Merker 13/25 X 2,688,070 8/1954 Freedlander 219/528 1 Dec. 25, 1973 2,799,765 7/1957 Jenkins et al. 219/464 2,913,695 11/1959 Borghult et al...... 13/25 X 3,086,101 4/1963 Scofield 219/464 X 3,413,132 ll/l968 Fishwick 106/40 R 3,432,580 3/1969 Heidrich et al. 106/40 R X 3,626,149 12/1971 Carney 219/213 Primary Examiner-Velodymyr Y. Mayewsky Att0rneyDavid E. Dougherty et a1.

[5 7] ABSTRACT An improved electrical heating unit useful as a surface cooking element and in a wide variety of other applications comprises a unitary fused silica body having a thermally conductive, normally upper layer consisting essentially of dense fused silica and, fused thereto, a thermally insulating, normally lower layer consisting essentially of closed cell cellular fused silica, one or more channels at the interface of the dense silica and cellular silica layers, and one or more electrical resistance heating elements disposed within said channels and preferably in contact with the dense silica layer.

13 Claims, 2 Drawing Figures PATENTEI] UEI225 I973 FIG. 2

I NVENTOR.

ROMUALD w. TYMCZAK BY fimf E. 511

1. ELECTRICAL HEATER BACKGROUND OF THE INVENTION The present invention relates to an electrical heater which is useful as a surface cooking element in electric ranges and in a wide variety of other applications.

Various types of electrical heating units are wellknown-in the art. One type of electrical heating unit which has found utility as a surface cooking element comprises a thermally conductive upper plate of metal, ceramic or glass, which serves as the cooking surface. Electrical resistance heating elements disposed below the cooking surface constitute the heat source. A lower layer, or backing, of a thermally insulating material is provided to restrict the downward loss of heat. The several'parts of such unitsmay be held together by mechanical means, or the thermally conductive upper layer and'the thermally insulating lower layer may be held'together by a refractory cement in which the heatingelements may be embedded.

Heating units of the type described are advantageous in-respect of high'thermal efficiency and ease of cleaning. However, they possess certain disadvantages which have prevented their wide-spread commercial acceptance: For example, they are relatively expensive inasmuch 'as considerable labor is involved in separately producing. and subsequently assembling the various parts. lfthe various parts are held together by mechanical means, the unit is generally not impervious to moistureand substances such as grease and fat which may be*spilled duringcooking, such materials being deleterious to the heating-element. When the several parts are held together by acement, the unit may be impervious to such materials at the outset, but the cement is subject'to thermal fatigue and ultimate cracking from repeated heating and cooling cycles, especially if the heating element is in contact with the cement, and the property of imperviousness is thus lost. Accordingly, the useful operating life of heating units of this type leaves much to be desired.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided an electrical heater of the general type described but which is markedly improved over prior heaters of this type, especially in that the upper thermally conductive layer and the lower thermally insulating layer are fused together to form a unitary body, thus eliminating the need for cement or mechanical means to hold-them together. The improved heater of the invention is characterized by simplicity of design, ease of fabrication and assembly, andcorrespondingly relative inexpensiveness. Such units may be easily produced in a widevariety of sizes and shapes and may be made of relatively light weight, thus being easily portable, if desired. They may easily be so constructed as to be impervious to extraneous substances, thus being characterized-by alongv operating lifeand outstanding reliability.

An electrical heater inaccordance with the invention comprises a unitary fused silica body having two layers. One of th'e'layers, normally the upper layer, consists essentially of dense fused silica and is therefore thermally conductive, electrically insulating, mechanical impactresistant, and impervious. The second layer, normally the lower layer, is a thermally insulating layer consisting essentially of closed cell cellular fused silica, this layer also being electrically insulating and impervious.

The two layers are fused together at their. interface, thereby forming a unitary body, the two layers preferably being fabricated simultaneously as a unitary body, as described hereinafter. One or more channels are disposed at the interface between the two layers, being substantially parallel to the plane of the interface, the channels being defined or bounded in part by the upper dense silica layer and in part by the lower cellular silica layer. One or more electrical resistance heating elements are provided, being disposed within the channels and preferably being in contact with the upper dense fused silica layer, to serve as a heat source.

BRIEF DESCRIPTION OF THE DRAWING The invention will be further described partly with reference to the accompanying drawing, wherein:

FIG. 1 is a perspective view ofa heating unit according to the invention, a portion of the fused silica layers DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawing, an electrical heater according to the invention is shown generally at 10. The heater comprises a unitary fused silica body 12. The unitary body 12 has a first layer 14 which is thermally conductive and which consists essentially of dense fused silica, and a second layer 16 which is thermally insulating and which consists essentially of closed cell cellular fused silica, the two layers 14 and 16 being fused together at their interface 18. Channels 20 are disposed at the interface 18, being substantially parallel to the plane of the interface and extending the entire length of the unitary body 12. The channels 20 are defined or bounded in part by the dense fused silica layer 14 and in part by the cellular fused silica layer 16, and

are preferably substantially parallel to each other and approximately equidistant from one another as shown. An electrical resistance heating element 24 in the form of a wire is disposed within the channels 20, extending the entire length of the channels 20, the two ends 28 and 30 of the heating element being electrically connected to any suitable source of electricity (not shown). The heating element 24 comprises a single continuous wire which passes back and forth through the neighboring channels 20, the element having hot portions or zones 32 of relatively high electrical resistance in the interior portions intermediate the ends of the channels 20, and having cold portions or zones 34 of relatively low resistance in and exterior to the ends of the channels 20. The hot portions 32 are preferably in contact with the dense fused silica layer 14 as shown in FIG. 2. The cold portions 34 may be held in place in the ends of the channels 20 by any suitable cement 36 such as a resin or an inorganic refractory cement, or by any other suitable means, which may also serve to prevent the ingress of moisture and other substances into the channels.

The two-layered, unitary fused silica body employed in the heating device of the present invention is preferably prepared by an adaptation of the method of pro ducing closed cell cellular fused silica described in detail in u.S. patent application Ser. No. 137,627 of Yorihiro Murata, filed April 26, 197], entitled Cellular Fused Silica and Production Thereof, which application is assigned to the assignee of the present application and the disclosure of which is incorporated herein by reference. In accordance with that application, boron oxynitride is employed as a cellulating agent for producing cellular silica, the boron oxynitride being prepared by heating boric acid in an ammonia atmosphere up to a final temperature within the range from about 700C to about l300C and continuing to heat at the final temperature until the desired boron oxynitride composition is obtained. A mixture is then prepared of finely divided silica and boron oxynitride, the mixture containing from about 0.1 percent to about percent by weight, preferably about 0.3 percent to about 3.0

percent and still more preferably about 0.5 percent boron oxynitride. the mix is heated to at least the melting point of the silica, whereupon the silica melts, the boron oxynitride decomposes to produce a gas which forms closed cells in the melt, resulting in a cellulated mass which forms a rigid body of closed cell cellular fused silica upon cooling. Preferably, the heating is carried out with the mix in a closed mold of the desired internal configuration which restricts free expansion of the mix during cellulation and thereby permits the bulk density of'the resulting cellular fused silica body to be controlled. Closed cell cellular fused silica bodies may thereby be produced having a bulk density within the range of from about 0.2 g/cc to about 1.2 g/cc or higher if desired, the compressive strength of the bodies increasing with increasing bulk density. The resulting cellular fused silica is of the closed cell type, i.e., the cells use inthe present electrical heating units by placing a layer of finely divided silica devoid of any cellulating agent in the bottom of the mold cavity prior to introducing the silica-boron oxynitride mixture. During the heating, the silica layer is converted to a dense fused silicalayer which, upon cooling, is found to be fused to the cellular silica layer.

EXAMPLE Finely divided boron oxynitride is intimately mixd with finely divided silica to obtain a substantially homogeneous mixture containing 1 percent boron oxynitride based upon the total weight of the mixture. The silica, of the quartzite crystal form, analyzes 99.6 percent SiO, and has a maximum particle size of about 50 microns, a mean particle size of about 8 microns, and a melting point of about 1680C.

A graphite mold is employed having inner dimensions 22.8 X 22.8 X 7.0 cm, the inner surfaces being covered with a smooth coating of boron nitride to prevent reaction of the silica with the graphite during firing. The mold is provided with a tightly fitting graphite cover, the inner surface of which is also covered with a smooth coating of boron nitride.

A layer of the same silica used in preparing the above mixture but not containing any boron oxynitride is placedin the bottom of the mold cavity, the layer bing 1.8 cm thick and 22.8 cm square. A 2000 gramportion of the silica-boron oxynitride mixture is evenly distributed over the silica layer and the mold is closed. The mold is then placed in a furnace and heated in a current of nitrogen to 1700C, whereupon the mold is removed and allowed to cool to room temperature, the resulting body then being removed from the mold. The unitary body comprises a layer of dense fused silica approximately 0.6 cm thick and 22.8 cm square fused to a layer of cellular fused silica having a bi-modal closed cell structure as described above which layer is approximately 6.4 cm thick and 22.8 cm square.

Cellular fused silica produced as in the example is readily machinable. Accordingly, channels to accommodate heating elements may readily be produced at the interface between the two layers of such unitary bodies by drilling or other conventional methods. The channels are preferably straight, extending from one end of the body to the other, and are conveniently of circular cross section, but may be of any other desired configuration and/or cross section. Instead of machining, the channels may be formed in the body at the time of firing by placing inserts at the interface between the two layers in the mold. Such inserts may be of any desired configuration and may, for example, be made of a metal having a melting point above the firing temperature, in which case the inserts are removed after firing and cooling, or the inserts may be made of a combustible material such as paper or plastic.

The dense fused silica layer is highly thermally conductive, electrically insulating, impervious to liquids and gases, and can be machined by conventional methods, if desired, to produce a very smooth surface. It is also characterized by good resistance to mechanical impact and by excellent thermal shock resistance, having an extremely low coefficient of thermal expansion. The thickness of the. layer may be varied depending upon the intended use for the heating unit, but a thickness of from about one-eighth inch to about one-fourth inch (about 0.3-0.6 cm) affords an approximately optimum balance between impact resistance and thermal conductivity.

The cellular fused silica layer is likewise impervious to gases and liquids and has an extremely low coefficient of thermal expansion, thus being extremely thermally shock resistant. The material is characterized by good mechanical strength which increases with increasing bulk denisty, and by low thermal conductivity which decreases with decreasing bulk density. The thickness of this layer and the bulk density thereof may be varied according to the intended use for the heating unit, depending upon the strength and the amount of thermal insulation required.

The two layers of the unitary bodies each consist essentially of fused silica, thus having the same coefficient of thermal expansion. Accordingly, no thermal stress occurs at the interface between the layers during heating and cooling of the heating units, and the body is not subject to failure from this cause. This is in marked contrast to heating units of the type wherein the thermally conductive layer and the insulating layer are joined by a cement, which may have a considerably different coefficient of thermal expansion from either or both of the layers and which therefore may render such units subject to thermal fatigue and failure.

Virtually any known type of electrical resistance heating element may be employed in the apparatus of the invention. For example, electrical resistance wire such as that which is commercially available under the trademark Nichrome may be used, as may conventional ceramic electrical resistance heating elements such as those composed of silicon carbide and commercially available under the trademark Globar. The elements may be straight, convoluted or of any other desired configuration. Preferably, the heating element has a hot portion in the interior region of the channel intermediate the ends thereof, and cold portions at the ends of the channel. The cold ends may thus be held in place by any of various resinous or other cementitious materials, or, since the unitary body is electrically insulating, by metal clamps. The hot portion of the element is preferably in contact with the dense silica layer to afford maximum thermal efficiency. One or more individual heating elements may be provided in each of a plurality of channels in the unitary body, or a single element may extend through a plurality of channels. The elements may be wired in any desired manner such that they may be individually or jointly controlled. In addition to means for turning the current to the heating elements on and off, means may also be provided to regulate the amount of current, thereby affording temperature control.

The heating units of the invention are extremely versatile. As surface cooking elements, they may be adapted to be plugged into the top of a home cooking range. They may be made of relatively light weight, thus being useful in portable hot plates or grills. They are readily adaptable to use as baking or broiling units in home ovens. They are also ideally suited to use in industrial heating apparatus such as ovens, since the units may be made impervious to industrial process atmospheres. By virtue of the unitary body consisting essentially of fused silica, units may be made which are capable of prolonged operation at temperatures up to about l000C. Accordingly, infrared heating elements may be employed to produce an infrared heater, the dense fused silica being a good transmitter of infrared radiation. Heating units may be made in accordance with the invention in any desired size and in any of a wide variety of desired shapes such as circular, square, rectangular and the like, depending upon the intended use. The units may be so fabricated that the heating elements are easily replacable, if desired.

while the invention has been described and illustrated herein with reference to certain examples and preferred embodiments, it is to be understood that various changes and modifications may be made by those skilled in the art without departing from the concept of the invention, the scope of which is to be determined by reference to the following claims.

I claim:

1. An electrical heater comprising; a fused silica body having a first layer and second layer fused together, said first layer being thermally conductive and consisting essentially of dense fused silica, said second layer being thermally insulating and consisting essentially of closed cell cellular fused silica; said fused silica body having a channel bounded in part by said first layer and in part by said second layer; and an electrical resistance heating element disposed within said channel and extending therefrom and having electrical terminals for connection to an electrical source of energy.

2. An electrical heater as set forth in claim 1 wherein said channel is straight and extends through the entire length of said body.

3. An electrical heater as set forth in claim 2 wherein said electrical resistance heating element is in the form of a wire.

4. An electrical heater as set forth in claim 2 wherein said electrical resistance heating element is a ceramic heating element.

5. An electrical heater as set forth in claim 4 wherein said ceramic is silicon carbide.

6. An electrical heater as set forth in claim 2 wherein said electrical resistance heating element comprises a wire extending through the entire length of said channel and having a hot central zone and cold ends, said hot zone being disposed intermediate the ends of said channel and said cold ends being disposed in the ends of said channel.

7. An electrical heater as set forth in claim 6 wherein said hot zone is in contact with said first layer.

8. An electrical heater as set forth in claim 2 wherein said electrical resistance heating element is a silicon carbide heating element extending through the entire length of said channel and having a hot central zone and cold ends, said hot zone being disposed intermediate the ends of said channel and said cold ends being disposed in the ends of said channel.

9. An electrical heater as set forth in claim 8 wherein said hot zone is in contact with said first layer.

10. An electrical heater as set forth in claim 1 having a plurality of said channels, a hot zone of an electrical resistance heating element being disposed within each of said channels.

11. An electrical heater as set forth in claim 10 wherein said channels are straight and extend through the entire length of said body and are substantially parallel to each other.

12. An electrical heater as set forth in claim 8 having a plurality of said channels substantially parallel to and equidistant from each other, and a silicon carbide heating element disposed in each of said channels.

13. An electrical heater comprising: a fused silica body having a first layer and second layer fused together, said first layer being thermally conductive and consisting essentially of dense fused silica, said second layer being thermally insulating and consisting essentially of closed cell cellular fused silica; said fused silica body having therein a plurality of channels bounded in part by said first layer and in part by said second layer, said channels being substantially parallel to and equidistant from each other and having an electrical resistance heating element disposed therein and extending therefrom and having electrical terminals for connection to an electrical source of energy, said element comprising a wire having a plurality of hot zones and a plurality of cold zones, a hot zone being disposed intermediate the ends of each of said channels and said cold zones being disposed in and exterior to the ends of said channels.

UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION .v

Patent No. 3,781 527 Dated December 25Ll97'3 Inventor(8) Romual d W. Tymczak It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' 0n the front page of The Patent, the name "Tymzcak" (each occurrence) should read "Tymczak".

Column 2, linei65, the term "u. 8.", should read U. S. Column 3, l ine 5 3 "mi xd" should read mixed Column 5, l ine 51 "whi le" should read Whi le Column 5, l ine 59, comprising; a" should read comprising: a

Signed and sealed this 6th day of August 197A.

(SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN .A'lzcesisiirlg Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION i.

P n 3.781527 Dated December 25. 1973 Inventor(s) Romuald Tymczak It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

0n the front page of The Patent, the name "Tymzcak" (each occurrence) should read "Tymczak".

Column 2, line65, the term 'u. 5.", should read U. S.

Column 3, l ine gflmi xd" should read mixed Column 5, line "while" should read While Columh 5, line 59, "comprising; a" should read comprising: a

Signed and sealed this 6th day of August 197 (SEAL) Attest:

McCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

2. An electrical heater as set forth in claim 1 wherein said channel is straight and extends through the entire length of said body.
 3. An electrical heater as set forth in claim 2 wherein said electrical resistance heating element is in the form of a wire.
 4. An electrical heater as set forth in claim 2 wherein said electrical resistance heating element is a ceramic heating element.
 5. An electrical heater as set forth in claim 4 wherein said ceramic is silicon carbide.
 6. An electrical heater as set forth in claim 2 wherein said electrical resistance heating element comprises a wire extending through the entire length of said channel and having a hot central zone and cold ends, said hot zone being disposed intermediate the ends of said channel and said cold ends being disposed in the ends of said channel.
 7. An electrical heater as set forth in claim 6 wherein said hot zone is in contact with said first layer.
 8. An electrical heater as set forth in claim 2 wherein said eLectrical resistance heating element is a silicon carbide heating element extending through the entire length of said channel and having a hot central zone and cold ends, said hot zone being disposed intermediate the ends of said channel and said cold ends being disposed in the ends of said channel.
 9. An electrical heater as set forth in claim 8 wherein said hot zone is in contact with said first layer.
 10. An electrical heater as set forth in claim 1 having a plurality of said channels, a hot zone of an electrical resistance heating element being disposed within each of said channels.
 11. An electrical heater as set forth in claim 10 wherein said channels are straight and extend through the entire length of said body and are substantially parallel to each other.
 12. An electrical heater as set forth in claim 8 having a plurality of said channels substantially parallel to and equidistant from each other, and a silicon carbide heating element disposed in each of said channels.
 13. An electrical heater comprising: a fused silica body having a first layer and second layer fused together, said first layer being thermally conductive and consisting essentially of dense fused silica, said second layer being thermally insulating and consisting essentially of closed cell cellular fused silica; said fused silica body having therein a plurality of channels bounded in part by said first layer and in part by said second layer, said channels being substantially parallel to and equidistant from each other and having an electrical resistance heating element disposed therein and extending therefrom and having electrical terminals for connection to an electrical source of energy, said element comprising a wire having a plurality of hot zones and a plurality of cold zones, a hot zone being disposed intermediate the ends of each of said channels and said cold zones being disposed in and exterior to the ends of said channels. 